The overall goal of this project is to examine longitudinally the developing phenotype of autism both at the behavioral level and at the neuropsychological level early in the evolution of the disorder: first at age 2 to 3 and again at age 4 to 5, the first two years after diagnosis. A group of very young children with autism, ages two and three will be compared to two groups of young children who share many of the behavioral characteristics seen in children with autism, a group with general developmental deficits and a group with Fragile X syndrome (FXS), as well as a group of typically developing children. These three diagnostic groups will be matched on age, developmental level, gender, and SES. The fourth, typically developing group, will be matched by SES, gender, and mental age. Data will be collected at two points in time, 24 months apart (approximately 12 months apart for the typically developing group), using developmentally appropriate measures for all the behavioral and neuropsychological variables being researched across all the projects in this project grant. These are: signs and symptoms of autism, imitation/praxis, affective functioning, intersubjectivity, and executive function. Thus, this study is a downward replication of Study 3 with an added longitudinal component, which will allow us to examine developmental trajectories as well as developmental profiles on the key variables of interest in all three groups of children. Using the FXS control group will allow comparison of the phenotype in autism to that of FXS, a condition which closely mirrors autism so that we may test what is specific to the phenotype of idiopathic autism early in its development.